World-wide, 41% of the population live in areas where malaria is transmitted, such as parts of Africa, Asia, Middle East, Central and South America, Hispaniola, and Oceania. Each year between 350 and 500 million cases of malaria occur worldwide, and over one million people die, most of them young children in sub-Saharan Africa. In areas of Africa with high malaria transmission, an estimated 990,000 people died of malaria in 1995. In 2002, malaria was the fourth cause of death in children in developing countries. In addition, malaria caused 10.7% of all children's deaths in developing countries.
Antimicrobial peptides (AMPs) represent a component of the innate immune system that provides resistance to a variety of pathogenic bacteria. AMPs have provided new leads for developing antibiotics, because they play a central role in the innate immune system. Some AMPs display very broad spectrum action against bacteria, yeast, fungus, and even viruses. Anti-parasitic activities have also been reported for a number of host defense peptides. The best studied organisms include Plasmodium, Leishmania, and Trypanosoma (Vizioli et al., Trends in Pharmacol., 2002, 18, 475-476; Jacobs et al., Antimicrob. Agents Chemother., 2003, 47, 607-613; and Brand et al., J. Biol. Chem., 2002, 277, 49332-49340), the parasitic agents of malaria, leishmaniasis and Chagas' disease, respectively. Additional protozoan parasites reported to be killed by the host defense peptides are Cryptosporidium (Giacometti et al., Antimicrob. Agents Chemother., 2000, 44, 3473-3475) and Giardia (Aley et al., Infect. Immun., 1994, 62, 5397-5403), human pathogens transmitted in contaminated drinking water. The peptides appear to kill protozoa by interacting with the cytoplasmic membrane causing excessive permeability, lysis and death; a mechanism which is similar to their mechanism of action against bacteria. Specificity for the parasite versus the host cell can be attributed to differences in phospholipid content and the lack of cholesterol in the protozoan membrane. Because the site of action is at the membrane and not to any specific receptor or intracellular target, the development of resistance to the cytotoxic properties of the antimicrobial peptides is highly unlikely.
With regard to anti-malarial activities, natural host defense proteins and their analogs have been shown to inhibit oocyst development of several Plasmodium species in various mosquito hosts (Gwadz et al., Infect. Immun., 1989, 57, 2628-2633; and Possani et al., Toxicon, 1998, 36, 1683-1692) and are directly cytotoxic against early sporogonic stages of Plasmodium in cell culture (Arrighi et al., Antimicrob. Agents Chemother., 2002, 46, 2104-2110). Furthermore, several antimicrobial peptides have been identified which selectively kill intraerthrocytic parasites (plasmodia life forms growing in red blood cells) by either attacking the infected erythrocyte while sparring normal erthrocytes (Feder et al., J. Biol. Chem., 2000, 275, 4230-4238; and Krugliak et al., Antimicrob. Agents Chemother., 2000, 44, 2442-2451) or interacting with and killing the intracellular parasite without harming the infected red blood cell (Dagan et al., Antimicrob. Agents Chemother., 2002, 46, 1059-1066; and Efron et al., J. Biol. Chem., 2002, 277, 24067-24072). Recognizing the significant therapeutic limitations of peptides, the development of nonpeptidic mimics of these anti-plasmodia peptides would represent a novel and powerful therapy to combat malaria.